Diesel engines, some gasoline fueled engines and many hydrocarbon-fueled power plants are operated at higher than stoichiometric air-to-fuel mass ratios for improved fuel economy. The hot exhaust gas produced by such lean-burn engines generally contains a relatively high concentration of oxygen (about one to ten percent by volume) and water, as well as unwanted gaseous emissions that may need to be converted to more innocuous substances before being discharged to the atmosphere. The gaseous emissions primarily targeted for abatement include carbon monoxide (CO), unburned and partially burned hydrocarbons (HC), and nitrogen oxide compounds (NOX). The NOX constituent in the exhaust gas produced by a lean-burn engine comprises mostly NO (greater than 90 mol %) with some NO2 (less than 10 mol %) and nominal amounts of N2O. To the extent that the hydrocarbon fuel contains sulfur, the exhaust gas may also contain sulfur dioxide (SO2).
Exhaust gas treatment systems that include specially catalyzed flow-through reactors are commonly used to effectively treat exhaust gas flows with variable concentrations of CO, HC and NOX. In general, these treatment systems—and the catalyst materials therein—are designed to promote (1) the oxidation of CO to CO2, (2) the oxidation of HC to CO2 and water, and (3) the reduction of NOX to N2 and water. However, the high amounts of oxygen in the exhaust gas produced by a lean-burn engine may inhibit the catalytic reduction of NOX to N2 in commercially-available treatment systems. But, it is found that when much of the NO in an NOX-containing exhaust gas flow is first oxidized to NO2, the NOX constituency can more readily be reduced to N2 and water by commercially-available NOX reduction catalysts. Accordingly, the subsequent NOX to N2 conversion efficiency of such treatment systems can be enhanced by incorporating an NO oxidation catalyst into the treatment system upstream of the NOX reduction catalyst.
The above identified co-pending patent application discloses mixtures of co-precipitated and calcined base metal oxides that may be prepared as catalysts and used for the effective oxidation of CO to CO2, NO to NO2 and HC to CO2 and water in an oxygen-rich exhaust flow produced by a lean-burn engine. In one embodiment, a ternary mixture of base metal oxides including manganese (Mn) oxide, cerium (Ce) oxide and zirconium (Zr) oxide is prepared for catalysis of such oxidation reactions. It is disclosed that the concentration of manganese in these base metal oxide mixtures may vary, for example from about five mole percent to about ninety mole percent, depending on the catalyst application.
In the above identified parent application, three exemplary binary particulate mixtures comprising MnOX and one of CeO2, ZrO2 and Y2O3 are prepared and used to promote the oxidation of NO, CO and C3 hydrocarbons in a synthetic oxygen and water containing gas stream. In these illustrative examples, each of the binary catalyst mixtures consist of equal molar parts MnOX and CeO2, ZrO2 or Y2O3. Such binary catalyst mixtures were shown to effectively oxidize NO, CO and low-molecular weight hydrocarbons in the synthetic oxygen and water containing gas stream. The oxidation reactions were found to be effective at temperatures in the range of about 150° C. to about 450° C. and at volumetric gas flow rates experienced in the exhaust flow from a diesel or lean-burn engine.